を (through, along, out of)
through, along, across; out of
Place + を + movement verb (歩く · 渡る · 出る · 降りる)
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With verbs of movement, を marks the space you move through or along, or the place you leave. It's still pronounced o.
Through, along, across: 公園を散歩します, "walk in the park"; この道をまっすぐ行ってください, "go straight along this road"; 橋を渡る, "cross the bridge".
Leaving: 家を出ます, "leave the house"; バスを降ります, "get off the bus"; 大学を卒業する, "graduate from university".
Compare に, which is the destination (バスに乗る, "get on the bus"), and で, which is where an action happens. 公園で散歩する is also heard, but を stresses moving through the space.
Examples
毎朝、公園を散歩します。
I walk in the park every morning.
八時に家を出ます。
I leave the house at eight.
この道をまっすぐ行ってください。
Go straight along this road.
橋を渡ると、駅があります。
Cross the bridge and you'll find the station.
次の駅でバスを降ります。
I get off the bus at the next stop.